Gold and silver quantum clusters in molecular containers and methods for their preparation and use

ABSTRACT

A composition includes a quantum cluster of Ag m  or Au n , one or more protector molecules; and a molecular cavity partially or wholly surrounding the quantum cluster. A method for preparing the quantum clusters includes adding a first amount of glutathione to a gold salt, a silver salt, or a mixture thereof to form a mixture; adding a reducing agent to the mixture to form a precipitate; and mixing the precipitate with a second amount of glutathione and a cyclodextrin to form a composition. Devices are prepared from the quantum clusters, and the devices may be used in methods of authentification of articles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage application of InternationalApplication No. PCT/IB2011/000260, filed on Feb. 14, 2011, which in turnclaims the benefit of India Patent Application No. 4036/CHE/2010, filedDec. 30, 2010, both of which are incorporated herein by reference intheir entirety for any and all purposes.

FIELD

The present technology generally relates to quantum clusters. Inparticular, the present technology refers to quantum clusters made ofgold or silver.

BACKGROUND

Quantum clusters are materials having very few atoms; with core sizes inthe sub-nanometer range and which exhibit novel properties. Compared tometal nanoparticles, quantum clusters do not have a continuous densityof states but are characterized by discrete electronic energy levels.Quantum clusters act as a bridge between atomic and nanoparticlebehaviors and therefore exhibit properties different from both these.

SUMMARY

In one aspect, a composition is provided including a quantum clusterwith Ag_(m), Au_(n), or Ag_(m)Au_(n) where m and n are from 2 to 100,one or more protector molecules and a molecular cavity partially orwholly surrounding the quantum cluster. In some embodiments, theprotector molecule is a thiol. In some embodiments, thiol isglutathione, cysteine, mercaptosuccinic acid, dimercaptosuccinic acid,phenylethane thiol and other aliphatic and aromatic thiols. In someembodiments, the molecular cavity includes a cyclodextrin, calixirane,or a crown ether.

In some embodiments, the composition is luminescent. In someembodiments, the luminescence of the composition is medium-dependent,liquid-solvent dependent or solvent vapor-dependent.

In one aspect, a method is provided including adding a first amount ofglutathione to a gold salt, a silver salt, or a mixture thereof to forma mixture. The method includes adding a reducing agent to the mixture toform a precipitate, and mixing the precipitate with a second amount ofglutathione and a cyclodextrin to form a composition. In someembodiments, the method includes having a molar ratio of the firstamount of glutathione to the amount of the gold salt, the silver salt,or the mixture thereof of from about 1:2 to about 1:8. In someembodiments, the precipitate comprises a) a quantum cluster of, Ag_(m),Au_(n), or Ag_(m)Au_(n); and b) glutathione, where m and n areindependently from 2 to 100. In some embodiments, m and n are from 2 to50. In some other embodiments, m and n are independently from 10 to 40.In some embodiments, the gold salt is a trivalent gold source. In someembodiments, the gold salt is HAuCl₄.3H₂O, AuCl₃, or a mixture thereof.In some embodiments, the reducing agent is NaBH₄, LiBH₄, or a mixturethereof.

In one aspect, a device is provided with a substrate; and a compositioncoated on the substrate; where the composition includes a quantumcluster, a protector compound, and a molecular cavity and the deviceexhibits solvent-dependent luminescence when exposed to a liquid solventor a solvent vapor. In some embodiments, the substrate comprises SiO₂,glass, conducting glass, quartz, silicon, or functionalized polymers.

In one aspect, a method is provided including the steps of providing asubstrate coated with a composition comprising a quantum cluster, aprotector compound, and a molecular cavity; exposing the substrate to afirst solvent; where the first solvent induces a luminescent responsefrom the composition; the composition is coated on the substrate in apattern; and the first solvent comprises a liquid solvent or solventvapor. In some embodiments, the methods includes removing the firstsolvent, wherein the composition does not luminesce, or exhibits aluminescence of reduced intensity, after the first solvent is removed.In some embodiments, removing the solvent includes exposing thesubstrate to a second solvent, wherein the second solvent does notproduce a luminescent response from the composition, and contacting thesubstrate with the second solvent quenches the luminescent responseinduced by the first solvent. In some embodiments, the removing includesallowing the first solvent to evaporate from the substrate. The methodcan further comprise detecting the presence or absence of theluminescent response.

In one aspect, a composition is provided with a core comprising 15 Auatoms, one or more glutathione molecules, and one or more cyclodextrinmolecules wherein the cyclodextrin molecules at least partially surroundthe Au atoms. In some embodiments, two molecules of cyclodextrinpartially or wholly surround the Au atoms. In some embodiments, thecyclodextrin is

,

, or

-cyclodextrin.

In one aspect, a labeling system is provided including a substratehaving a coating comprising a quantum cluster, a protector compound, anda molecular cavity. The coating on the substrate is in a pattern and thepattern is luminescent when exposed to solvent. The labeling systemincludes a means for detecting the pattern on the substrate. In someembodiments, the pattern includes letters, numbers, symbols, pictures,or barcodes. In some embodiments, the substrate is attached to currency,financial and legal documents, shipping containers, electronics, medicaldevice, pharmaceutical packaging, packaging on consumer items orbiological compounds.

In one aspect, a method of authentication is provided including exposinga patterned coating to a solvent and detecting the patterned coating. Insome embodiments, the coating includes a quantum cluster, a protectorcompound, and a molecular cavity. In some embodiments, the coating isluminescent when exposed to the solvent and luminescence intensity isreduced when the solvent is removed. In some embodiments, the patternedcoating encodes the authenticity of an object to which the coating isapplied. In some embodiments, the patterned coating comprises letters,numbers, symbols, pictures, or barcodes. In some embodiments, the objectis currency, a financial document, a legal document, a shippingcontainer, an electronic object, an envelope, or packaging. The methodcan include detecting the presence or absence of the patterned coatingon the object. In some embodiments, the method includes verifyingauthenticity of the object by correlating the presence of the patternedcoating with authenticity of the object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the process for making the cluster composition,according to one embodiment.

FIG. 2 is a schematic showing the interaction of the SG ligand of thecluster with the CD molecule, according to one embodiment.

FIG. 3A is the UV-Vis spectra of Au₁₅@αCD, Au₁₅@βCD and Au₁₅@γCDclusters, according to the examples.

FIG. 3B is the natural logarithm of Jacobian factor-corrected absorbanceversus the wavelength of Au₁₅@αCD, Au₁₅@βCD, and Au₁₅@γCD, according tothe examples.

FIG. 3C is a graph of the absorption profiles of pure GSH and α-CD,according to the examples.

FIG. 3D is a graph of the luminescence spectra of three of the quantumcompositions—Au₁₅@αCD; Au₁₅@βCD; Au₁₅@γCD, according to the examples.

FIG. 4A is the circular dichroism spectra of Au₁₅@αCD, Au₁₅@βCD Au₁₅@γCDclusters along with pure GSH and Au₂₅SG₁₈, according to the examples.

FIG. 4B shows the combined plot of absorption and circular dichroism(CD) spectra for Au₁₅@αC, according to the examples.

FIG. 5 is a luminescence spectra of a TLC plate coated with Au@CD, whencontacted with various solvents, according to the examples.

FIG. 6A is an EDAX spectrum of the gel formed by cluster compositions,according to the examples.

FIG. 6B is an SEM image of the gel formed by the cluster compositions,according to the examples.

FIG. 6C is an EDAX mapping of the gel using Au_(n), corresponding to SEMimage in 6A, according to the examples.

FIG. 6D is a TEM image of the gel shows the self assembly and fiber-likemorphology, according to the examples.

FIGS. 7A & 7B are UV-Vis spectra of Au₁₅@αCD cluster before and afterthe addition of Cu²⁺, according to the examples.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part thereof. The illustrativeembodiments described in the detailed description, drawings and claimsare not meant to be limiting. Other embodiments may be utilized, andother changes may be made, without departing from the spirit or scope ofthe subject matter presented here. The present technology is alsoillustrated by the examples herein, which should not be construed aslimiting in any way.

In general, compositions are provided which include Ag_(m), Au_(n), orAg_(m)Au_(n) quantum clusters are provided by combining host-guestchemistry with core-etching. Such compositions exhibit luminescence thatis dependent upon the environment in which the quantum cluster islocated. Such compositions may also be useful in authentificationprocesses. The compositions, devices, and methods are all described ingreater detail below. As used herein, “core etching” refers to treatmentof the metal quantum clusters with excess of protector molecules. Asused herein, “host-guest chemistry” refers to partial or wholecontainment of the metal quantum clusters in the molecular cavity.

In one aspect, compositions including Ag_(m), Au_(n), or Ag_(m)Au_(n)quantum clusters are provided. In some embodiments, the compositionincludes one or more protector molecules surrounding the metal atoms,and m and n are from 2 to 100. In some embodiments, n is from 5 to 50,or from 10 to 20. In some embodiments, n is 15. Specific examples of ninclude 2, 3, 4, 5, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, andranges between any two of these values. In some embodiments, the quantumcluster comprises Au_(n). In other embodiments, the quantum clustercomprises Ag_(m).

In some embodiments, the protector molecule is a thiol. The protectormolecule can bind to the surface of the quantum cluster, thereby forminga layer partially or fully around the cluster and protecting thecluster. In some embodiments, the protector molecules may includeglutathione (GSH), cysteine, homocysteine, mercaptosuccinic acid,dimercaptosuccinic acid, phenylethane thiol, or other alkyl or arylthiols. Generally, where the protector molecule is a mono thiol, it willreduce the Au³⁺ to Au¹⁺. Further reduction may be carried out by theaddition of a reducing agent. In contrast, where the protector moleculeis a dithiol, for example the dimercaptosuccinic acid, one of the thiolgroups will act as reducing agent (i.e. one of the groups is oxidized)and the other will act as a protecting agent (i.e. it is reduced). Insome embodiments, the protector molecule is a reduced form of theprotector molecule; such as reduced glutathione. Clusters of Au atomswith the glutathione ligand (-SG) may be represented as Au@SG forconvenience.

In some embodiments, a molecular cavity partially or wholly surroundsthe metal cluster with the protector molecule, e.g., Au@SG. Suchmolecular cavities may include one or more molecules of cyclodextrin,calixirane, or a crown ether. In addition, it may be possible to createcluster compositions within other molecular cavities such as PAMAM, BSA,and the like.

Cyclodextrins are a family of compounds made up of sugar molecules boundtogether in a ring. In some embodiments, α-, β-, or γ-cyclodextrin (α-,β-, or γ-CD) molecules are used to form the cluster compositionrepresented as Au@αCD, Au₁₅@βCD or Au₁₅@γCD. Cyclodextrins arebowl-shaped molecules which may “trap” or “encapsulate” other moleculesof an appropriate size which will fit into the bowl.

As noted, the above compositions exhibit luminescence. Such luminescencemay be medium-dependent, liquid solvent-dependent, or solventvapor-dependent. For example, when the composition is exposed to somemedia, the luminescence is increased. In some embodiments, the solventis, but not limited to, an alcohol, methanol, ethanol, or propanol;water; acetonitrile; acetone; dichloromethane; carbon tetrachloride;chloroform; toluene; hexane; or a mixture of any two or more thereof. Insome cases, it is observed that the more hydrogen bonding between thecluster ligands and the solvent, the greater the luminescence.Accordingly, the luminescence of the composition in the presence ofeither methanol or ethanol is greater than that for propanol. In someembodiments, where the composition is placed in water, luminescentemission at 318 nm, 458 nm and 580 nm is observed.

In another aspect a method of preparing the compositions is provided. Inone embodiment, a gold or silver salt is mixed with the thiol protectormolecule to precipitate a protected metal cluster. In some embodiments,where it is a gold slat that is used, and the protector molecule isglutathione, the cluster particles (referred to as Au@SG) may beprecipitated from the mixture in the presence of a reducing agent. It isunderstood that gold salts such as HAuCl₄.3H₂O, AuCl₃, or othertrivalent gold salts may be used in the methods. Suitable reducingagents include, but are not limited to NaBH₄ and LiBH₄, as well as otherknown reducing agents, or a mixture of any two or more reducing agents.In some embodiments, the amount of gold or silver salt to protectormolecule ranges from about 1:2 to about 1:8. In other embodiments, themetal cluster has formula Ag_(m), Au_(n), or Ag_(m)Au_(n) where m and nare from 2 to 100. In some embodiments, n is from 5 to 50, or from 10 to20. Specific examples of n include 2, 3, 4, 5, 10, 15, 20, 25, 30, 40,50, 60, 70, 80, 90, 100, and ranges between any two of these values. Insome embodiments, n is 15. In some embodiments, n is from 5 to 50, orfrom 10 to 20. Specific examples of n include 2, 3, 4, 5, 10, 15, 20,25, 30, 40, 50, 60, 70, 80, 90, 100, and ranges between any two of thesevalues. In some embodiments, n is 15.

After preparation of the protected metal cluster, it is then mixed withthe molecule that is to form the molecular cavity to form thecomposition of the protected metal cluster partially or fully surroundedby the molecular cavity. For example, where the protected metal clusteris Au@SG, it may be processed with an α-, β-, or γ-cyclodextrin (CD)molecule to form the cluster composition, Au₁₅@CD. In one embodiment,the Au@SG precipitate is dissolved in aqueous solution and mixed with CDand excess GSH. The Au₁₅@CD cluster composition may be collected fromthe solution by any known method of separation. Such separation mayinclude centrifugation in some embodiments.

In another aspect, a device is provided which includes any one of theabove compositions deposited on a substrate. Suitable substratesinclude, but are not limited to SiO₂, glass, conducting glass, quartz,silicon or functionalized polymers. For example, in one embodiment, thesubstrate is a thin layer chromatography (TLC) plate coated with a SiO₂stationary phase. In other embodiments, the substrate is a chitosan, acarbon nanotube, activated carbon, alumina, or the like. Such substratesmay be sequestered on a surface, or the substrate may be a powder orsuspension. In some embodiments, the cluster compositions are uniformlycoated on the substrate. In some embodiments, the cluster compositionsare coated on the substrate in a pattern. The devices coated with thecompositions exhibit a solvent- or medium-dependent luminescence asdescribed above for the compositions.

Any solvent may be used to produce luminescence including methanol,ethanol, 2-propanol, water, acetonitrile, acetone, dichloromethane,carbon tetrachloride, chloroform, toluene, hexane or a mixture. As usedherein, “solvent” may be a liquid solvent or solvent vapors.

In some embodiments, the luminescence of the substrate may be reduced oreliminated when the solvent is removed by either evaporation of thefirst solvent or exposure to a second solvent. In some embodiments,exposure to a first solvent produces luminescence of a clustercomposition coated substrate and exposure to a second solvent quenchesthe luminescence. In some embodiments, the intensity of the luminescenceis a dependant on the concentration of the solvent. In some embodiments,a device which includes a cluster composition coated substrate may beused to monitor or identify solvent vapors in the air. The clustercomposition may also be used for the selective detection of metal ionsas described in Example 8.

Useful information about a product may be contained in a label includingthe cluster compositions. In some embodiments, a labeling system isprovided having a substrate coated with the cluster compositions in apattern and the pattern is luminescent when exposed to solvents. Thelabeling system may also include a detection system or means fordetecting the pattern on the substrate, or it may be observed visually.The pattern on the substrate may include letters, numbers, symbols,pictures or barcodes that can capture information. The substrate may beattached to currency, financial and legal documents, shippingcontainers, electronics, medical device, pharmaceutical packaging,packaging on consumer items, biological compounds or other items thatshould be labeled. For example, the composition may be used to label anarticle or an object, either with a physical label, or through directapplication of the composition to form a label. In some embodiments, thelabel is invisible to naked eye, but when exposed to a substance, suchas a solvent liquid or vapor, the luminescence of the composition isaltered, and the label may be visualized.

The detection system or “means for detecting” may be a device that cancapture the luminescent pattern including a camera, a UV-Vis detector, ascanner, and the like. As used herein, “coating” of the clustercompositions is understood to include attachment, embedment or othermeans of binding or association.

In some embodiments, the cluster composition is used in a method forauthentication. The cluster composition may be coated on an object in aparticular pattern where the authenticity of the object is encoded inthe pattern. When the object is exposed to a solvent, the patternbecomes luminescent and the luminescence intensity is reduced when thesolvent is removed. In some embodiments, the, pattern will be detectedwith a device such as a camera, UV-Vis detector or the human eye uponcontact with the solvent, but the pattern is not detectable, or it isinvisible to the naked eye in the absence of the solvent.

In some embodiments, the object may be any item that may needauthentication such as currency, financial or legal document, a shippingcontainer, an electronic object, medical device, pharmaceuticalpackaging, an envelope, packaging or other item. In some embodiments,authenticity of the object will be verified by checking the presence orabsence of the pattern of luminescence. Thus, in one embodiment, amethod of authentication includes exposing a patterned coating to asolvent, where the patterned coating includes a composition as describedabove. The method also includes detecting the patterned coating. In suchmethods, the coating is luminescent when exposed to the solvent and isnot luminescent when the solvent is removed; and the patterned coatingencodes the authenticity of an object to which the coating is applied.The method may also then include verifying the authenticity of theobject. Such authentifications may include the patterned coating beingin the form of letters, numbers, symbols, pictures, or barcodes. Thus,the verification may be based upon coding, a standard message, or thelike.

As used herein, the following definitions of terms shall apply unlessotherwise indicated.

As used herein, “about” will be understood by persons of ordinary skillin the art and will vary to some extent depending upon the context inwhich it is used. If there are uses of the term which are not clear topersons of ordinary skill in the art, given the context in which it isused, “about” will mean up to plus or minus 10% of the particular term.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the elements (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. Recitation of ranges of values herein are merely intended toserve as a shorthand method of referring individually to each separatevalue falling within the range, unless otherwise indicated herein, andeach separate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate the embodiments and does not pose alimitation on the scope of the claims unless otherwise stated. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential.

The embodiments, illustratively described herein may suitably bepracticed in the absence of any element or elements, limitation orlimitations, not specifically disclosed herein. Thus, for example, theterms “comprising,” “including,” “containing,” etc. shall be readexpansively and without limitation. Additionally, the terms andexpressions employed herein have been used as terms of description andnot of limitation, and there is no intention in the use of such termsand expressions of excluding any equivalents of the features shown anddescribed or portions thereof, but it is recognized that variousmodifications are possible within the scope of the claimed technology.Additionally, the phrase “consisting essentially of” will be understoodto include those elements specifically recited and those additionalelements that do not materially affect the basic and novelcharacteristics of the claimed technology. The phrase “consisting of”excludes any element not specified.

As used herein, “thiols” are compounds with an “SH” functional grouprepresented by R-SH where R may be H, an alkyl, or aryl group.

Alkyl groups include straight chain, branched chain, or cyclic alkylgroups having from 1 to 20 carbon atoms or, in some embodiments, from 1to 12, 1 to 8, 1 to 6, or 1 to 4 carbon atoms. Examples of straightchain alkyl groups include those with from 1 to 8 carbon atoms such asmethyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, andn-octyl groups. Examples of branched alkyl groups include, but are notlimited to, isopropyl, iso-butyl, sec-butyl, tert-butyl, neopentyl,isopentyl, and 2,2-dimethylpropyl groups. Representative substitutedalkyl groups may be substituted one or more times with substituents suchas those listed above. Where the term haloalkyl is used, the alkyl groupis substituted with one or more halogen atoms.

Cycloalkyl groups are cyclic alkyl groups such as, but not limited to,eyelopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, andcyclooctyl groups.

In some embodiments, the cycloalkyl group has 3 to 8 ring members,whereas in other embodiments the number of ring carbon atoms range from3 to 5, 3 to 6, or 3 to 7. Cycloalkyl groups further include mono-,bicyclic and polycyclic ring systems, such as, for example bridgedcycloalkyl groups as described below, and fused rings, such as, but notlimited to, decalinyl, and the like. In some embodiments, polycycliccycloalkyl groups have three rings. Substituted cycloalkyl groups may besubstituted one or more times with, non-hydrogen and non-carbon groupsas defined above. However, substituted cycloalkyl groups also includerings that are substituted with straight or branched chain alkyl groupsas defined above. Representative substituted cycloalkyl groups may bemono-substituted or substituted more than once, such as, but not limitedto, 2,2-, 2,3-, 2,4- 2,5- or 2,6-disubstituted cyclohexyl groups, whichmay be substituted with substituents such as those listed above.

Aryl, or arene, groups are cyclic aromatic hydrocarbons that do notcontain heteroatoms. Aryl groups include monocyclic, bicyclic andpolycyclic ring systems. Thus, aryl groups include, but are not limitedto, phenyl, azulenyl, heptalenyl, biphenylenyl, indacenyl, fluorenyl,phenanthrenyl, triphenylenyl, pyrenyl, naphthacenyl, chrysenyl,biphenyl, anthracenyl, indenyl, indanyl, pentalenyl, and naphthylgroups. In some embodiments, aryl groups contain 6-14 carbons, and inothers from 6 to 12 or even 6-10 carbon atoms in the ring portions ofthe groups. Although the phrase “aryl groups” includes groups containingfused rings, such as fused aromatic-aliphatic ring systems (e.g.,indanyl, tetrahydronaphthyl, and the like), it does not include arylgroups that have other groups, such as alkyl or halo groups, bonded toone of the ring members. Rather, groups such as tolyl are referred to assubstituted aryl groups. Representative substituted aryl groups may bemono-substituted or substituted more than once. For example,monosubstituted aryl groups include, but are not limited to, 2-, 3-, 4-,5-, or 6-substituted phenyl or naphthyl groups, which may be substitutedwith substituents such as those listed above.

In general, “substituted” refers to a group, as defined above (e.g., analkyl or aryl group) in which one or more bonds to a hydrogen atomcontained therein are replaced by a bond to non-hydrogen or non-carbonatoms. Substituted groups also include groups in which one or more bondsto a carbon(s) or hydrogen(s) atom are replaced by one or more bonds,including double or triple bonds, to a heteroatom. Thus, a substitutedgroup will be substituted with one or more substituents, unlessotherwise specified. In some embodiments, a substituted group issubstituted with 1, 2, 3, 4, 5, or 6 substituents. Examples ofsubstituent groups include: halogens (i.e., F, Cl, Br, and I);hydroxyls; alkoxy, alkenoxy, alkynoxy, aryloxy, aralkyloxy,carbonyls(oxo), carboxyls, esters, urethanes, thiols, sulfides,sulfoxides, sulfones, sulfonyls, sulfonamides, amines, isocyanates,isothiocyanates, cyanates, thiocyanates, nitro groups, nitriles (i.e.,CN), and the like.

The present technology, thus generally described, will be understoodmore readily by reference to the following example, which is provided byway of illustration and is not intended to limit the present technology.

EXAMPLES Example 1

Synthesis of Au@SG. To a 50 mL methanolic solution (0.5 mM) ofHAuCl₄.3H₂O, 1.0 mM GSH was added (1:2 molar ratio, total volume ofmethanol was 50 mL). The mixture was cooled to 0° C. in an ice bath for30 minutes. An aqueous solution of NaBH₄ (0.2 M, 12.5 mL), cooled to 0°C., was injected rapidly into the above mixture under vigorous stirring.The mixture was allowed to react for another hour. The resultingprecipitate was collected and washed repeatedly with methanol throughcentrifugal precipitation. Finally, the Au@SG precipitate was dried andcollected as a dark brown powder. The size of Au@SG particles was in therange of 2-3 nm.

Example 2

Cyclodextrin assisted synthesis of Au₁₅ clusters. The abovenanoparticles (50 mg) were dissolved in 40 mL of de-ionized watercontaining 1.6 mole of GSH and 2.2×10⁻⁴ mole of cyclodextrin (the threeCD molecules were used separately). The mixture was heated at 70° C. for48 hours. The completion of the reaction was monitored by checking thered emission of the cluster under UV light. Intense red emission fromthe sample indicates the formation of the desired cluster. The entiresolution was centrifuged at 5000 rpm for 10 minutes. The whitish brownprecipitate of Au(1)thiolate was discarded. The supernatant was thentransferred to a plastic vial and freeze dried to obtain a brown powderwith intense red emission in the solid state. The same method was usedfor all the three CD molecules (α-, β-, and γ-cyclodextrin) resulting inthree separate cluster products. The material was washed twice withethanol to remove excess GSH. Analysis was done with energy dispersiveanalysis of X-rays (EDAX). The sample becomes a gel if the solution isallowed to dry in air.

FIG. 1 is a schematic of the process for synthesizing some embodimentsof the cluster compositions. A Au nanoparticle 10 interacts with one ormore protector molecules 20. In some embodiments, the protectormolecules are glutathione (-SG) ligands. The Au cluster 10 and the -SGligand 20 form the Au@SG particle 25. The Au@SG clusters are then mixedwith a molecular cavity 30, which may be one or more cyclodextrin (CD)molecules, in the presence of reduced glutathione molecules (GSH) 40. Asshown in FIG. 1, various cluster compositions may form during thisprocess including—the Au cluster is partially or wholly surrounded byone CD molecule as in 50, which is an illustration of an Au₁₅SG₁₃@CDcavity. It is believed that the -SG ligand 20 interacts with the CDmolecules 30 (partially shown) in the cluster composition 50 as shown inFIG. 2A. In particular, the proton ‘e’ of the -SG ligand 20 interactswith the ‘H3’ proton of the CD molecule 30. This was confirmed by 2D ¹HNMR (ROESY) of Au₁₅@αCD, which showed cross peaks for the H3 and eprotons. ¹H NMR of Au₁₅@αCD suggests that the -SG ligands near thesurface of the cluster compositions are in two differentenvironments—inside or outside the CD molecules. In addition, the -SGpeaks are shifted from the peak for the parent GSH suggesting that thereare no free GSH molecules.

Example 3

UV-Vis spectra were recorded using a Perkin Elmer Lambda 25spectrophotometer. The experimentally obtained intensities inabsorbance, as a function of wavelength [I(W)], have been converted toenergy-dependent values [I(E)] using the expression I(E)=I(W)/(∂E/∂W)αI(W)×W², where ∂E/∂w represents the Jacobian factor. The photoexcitationand luminescence, studies were done using a NanoLog HORIBA JOBINYVONspectrofluorimeter with a 100 W xenon lamp as the excitation source, ata scan speed of 240 nm/sec. Band pass for both excitation and emissionmonochromators was kept at 5 nm. Metal ion detection was studied at ppmconcentrations. Acetates (Cu²⁺ and Hg²⁺), nitrates (Ag¹⁺, Cd²⁺ and Zn²⁺)and chlorides (Fe³⁺) were used for metal ion detection studies. XPSmeasurements were done using an Omicron Nanotechnology spectrometer withpolychromatic Al K_(α) X-rays (hv=1486.6 eV). At least ten spectra inthe desired binding energy range were collected and an average wastaken. The samples were spotted as drop cast films on the sample stuband dried under vacuum. X-ray flux was adjusted to reduce the beaminduced damage of the sample. The energy resolution of the spectrometerwas set at 1.1 eV, at a pass energy of 50 eV. Binding energy (BE) wascalibrated with respect to C1s at 285.0 eV. Luminescence transients weremeasured and fitted using a commercially available spectrometer(Lifespec-ps) from Edinburgh instrument, U.K. (80 ps instrument responsefunction (IRF)). ¹H NMR and 2D ¹H NMR (ROESY) spectra were measured witha 500 MHz Brüker Advance III spectrometer operating at 500.13 MHz for ¹HNMR and equipped with a 5 mm triple-resonance PFG probe. Solutions weremade in 99.98% D₂O (Aldrich) and sealed immediately. The signal of thesolvent served as the reference for the field-frequency lock. Allexperiments were performed at a temperature of 25° C. unless specified.Standard Bruker pulse programs (Topspin 2.1) were employed throughout.The 1D spectra were acquired with 32 K data points. The data for phasesensitive ROESY experiments were acquired with a sweep width of 5600 Hzin both dimensions. For each spectrum, 16 transients of 2048 complexpoints were accumulated for 256 t₁-increments and a relaxation delay of2s was used. A CW spin lock mixing time of 200 ms was employed. Prior toFourier transformation, zero filling to 2K×2K complex points wasperformed, and apodized with a weighted function (QSINE) in bothdimensions. All the data were processed on a HP workstation usingTopspin 2.1 software. Mass spectrometric studies were conducted using anelectrospray (ESI-MS) system, 3200 Q-TRAP LC/MS/MS (Applied Biosystems).Samples of 15 ppm concentration, taken in 1:1 water/methanol mixturewere electrosprayed at a flow rate of 10 μL/min and ion spray voltage of5 kV. Circular dichroism studies were measured using a JASCO J-810circular dichroism spectropolarimeter. Limit of detection (LOD) of GSHin circular dichroism is approximately 0.1 mg/mL. The opticalpolarization image was measured using a Nikon Eclipse LV 100 POLpolarizing microscope. Dynamic light scattering (DLS) measurements werecarried out with Nano-S Malvern-instrument employing a 4 mW He-Ne laser(λ=632.8 nm) equipped with a thermostated sample chamber. All thescattered photons were collected at 173° scattering angle. Thescattering intensity data were processed using the instrumental softwareto obtain the hydrodynamic diameter (d_(H)) and the size distribution ofthe scatterer in each sample.

Example 4

Dynamic light scattering (DLS) measurements were performed to understandthe size of the cluster compositions in solution. DLS measurements werecarried out with Nano-S Malvern-instrument employing a 4 mW He—Ne laser(λ=632.8 nm) equipped with a thermostated sample chamber. All thescattered photons were collected at 173° scattering angle. Thescattering intensity data were processed using the instrumental softwareto obtain the hydrodynamic diameter (d_(H)) and the size distribution ofthe scatterer in each sample.

The hydrodynamic diameter of the cluster compositions were observed tobe 3-4 nm, which implies the presence of one cluster per CD moleculewith water of hydration (see 50 in FIG. 1).

Although the structure of Au₁₅ core is not available from single crystalXRD studies, calculations suggest it to have a C_(2ν) symmetric,shell-like flat cage structure with a pointed tip. It is possible that apart of the core or monolayers can penetrate into the CD cavity. Theinner core diameter is in the range of 0.6-0.9 nm for α, β or γ CDs,respectively, sufficiently large enough to partially accommodate Au₁₅cluster compositions.

The nature of the metal core in these cluster compositions was confirmedby XPS analysis. XPS measurements were done using an OmicronNanotechnology spectrometer with polychromatic Al K_(α) X-rays(hν=1486.6 eV). At least ten spectra in the desired binding energy rangewere collected and an average was taken. The samples were spotted asdrop cast films on the sample stub and dried under vacuum. X-ray fluxwas adjusted to reduce the beam induced damage of the sample. The energyresolution of the spectrometer was set at 1.1 eV, at a pass energy of 50eV. Binding energy (BE) was calibrated with respect to C1s at 285.0 eV.The 4f_(7/2) and 4f_(5/2)BEs of Au in all these cluster compositionsappear at 85.2 and 89.2 eV.

The S 2p, N 1s and C 1s core level spectra of these cluster compositionswere also measured. The Au/S atomic ratio measured from XPS is 1.150,which is in agreement with a composition of Au₁₅S₁₃ (theoretical valueis 1.1538). The Au₁₅ core reported is with 13 -SG ligands. The S 2poccurs at a slightly higher BE (163.1 eV) than typical thiolates (˜162.0eV) suggesting that the -SG protection is intact. The N is spectrumshows two peaks at 399.5 eV and 401.3 eV BE, indicating the presence of—NH and —NH₃ ⁺, respectively.

Example 5

Three samples of cluster compositions—Au₁₅@αCD; Au₁₅@βCD; Au₁₅@γCD wereprepared according to Example 2 each with α, β or γ CDs. FIG. 3A is aUV-Vis spectra of Au₁₅@αCD, Au₁₅@βCD, and Au₁₅@γCD. FIG. 3A indicatesthat the absorption wavelengths for Au₁₅@αCD, Au₁₅@βCD, and Au₁₅@γCDexhibit the same features. The distinct features of the Au₁₅ core areindicated by the ellipses. The UV-Vis spectra were recorded using aPerkin Elmer Lambda 25 spectrophotometer. The experimentally obtainedintensities in absorbance, as a function of wavelength [I(W)], have beenconverted to energy-dependent values [I(E)] using the expressionI(E)=I(W)/(∂E/∂W)α I(W)W×W², where ∂E/∂w represents the Jacobian factor.

The three varieties of the quantum composition have characteristicabsorption features at 318, 458 and 580 nm, where there are no featuresfor GSH as well as CD. FIG. 3B gives the plot of the natural logarithmof the Jacobian factor versus the wavelength of all the three clustersto show the molecular features more clearly (well-defined absorptionfeatures are marked by arrows). FIG. 3C provides the absorption profilesof pure GSH and α-CD.

FIG. 3D is the luminescence spectra of quantum compositions Au₁₅@αCD,Au₁₅@βCD, and Au₁₅@γCD. The photoexcitation and luminescence studieswere done using a NanoLog HORIBA JOBINYVON spectrofluorimeter with a 100W xenon lamp as the excitation source, at a scan speed of 240 nm/sec.Band pass for both excitation and emission monochromators was kept at 5nm. The samples were excited at 375 nm and the emission was observed at690 nm. These values are consistent with reported numbers for such asAu₂₂, Au₂₃, and Au₁₀.

Lifetime values of the clusters were obtained by numerical fitting ofthe luminescence at 690 nm. They are 0.029 ns (83.50%), 1.50 ns (5.90%),14.80 ns (2.60%) and 181 ns (8.0%) for Au₁₅@αCD; 0.071 ns (76.6%), 1.15ns (11.8%), 11.10 ns (4.5%) and 163 ns (7.1%) for Au₁₅@βCD and 0.024 ns(84.7%), 1.23 ns (7.6%), 13.90 ns (3.0%) and 172 ns (4.7%) for Au₁₅@γCD.The fast lifetime component is present in several clusters investigatedso far which also show an extremely slow component with reduced weight.For example, the Au₂₂ system shows a fast life time component of 0.05 ns(86.50%) and a slow component of 141.80 ns (3.40%).¹⁴ The quantum yieldsof the cluster compositions were approximately 6.7% (Au₁₅@αCD), 6.5%(Au₁₅@βCD) and 7% (Au₁₅@γCD) at room temperature, using ethidium bromideas the reference. In comparison to other similar clusters such as Au₂₂(4%) and Au₂₃ (1.3%), the quantum yield for the cluster compositionshere are substantially larger.

FIG. 4A shows circular dichroism spectra for the cluster compositionswith α, β and γ-CDs. Circular dichroism was measured using a JASCO J-810circular dichroism spectropolarimeter. Limit of detection (LOD) of GSHin circular dichroism is approximately 0.1 mg/mL. In addition, thespectra for pure GSH another cluster Au₂₅SG₁₈ have been included in FIG.4A as a comparison. GSH is a chiral compound with a negative Cotton peakaround 237 nm. The absence of this peak in the Au₂₅SG₁₈ cluster and thecluster composition suggest that there is no free GSH.

As seen in FIG. 4A, the spectrum of the cluster compositions aredifferent from the spectra for the Au₂₅SG₁₈ cluster. FIG. 4A suggeststhat the cluster compositions exhibit induced circular dichroism. Thecluster compositions have a positive Cotton peak around 330-380 nm and anegative Cotton peak around 400-455 nm, which may be attributed to thecluster core. FIG. 4B shows the combined plot of absorption and circulardichroism (CD) spectra for Au₁₅@αCD.

Example 6

Au₁₅@αCD was stored in a glass vial for 24 hours and the solution wasdecanted. The glass vial retains a thin layer of cluster compositioneven with water sonication for 10 minutes. The cluster compositionsremain intact and coat on the glass. Although not bound by theory, it isbelieved that the cluster compositions may bind with the Si—OH of theglass.

A thin layer chromatography (TLC) plate with bulk SiO₂ coating wascoated with the cluster composition solution of two differentconcentrations. The plate with the high and low concentration of clustercomposition shows red and rose emissions, respectively, under UV light.The cluster composition-coated TLC plate can be used as a substrate forchecking the solvent dependency of photoluminescence. The emission fromthe TLC plate was collected with 375 nm excitation. Then 2-propanol wassprayed over the plate using a sprayer. Immediately after spraying,emission was collected using the same excitation. There is a slightenhancement of the luminescence intensity. The plate was then allowed todry completely resulting in the reversal of luminescence. A few otheralcohols such as methanol and ethanol were sprayed. Luminescenceincreased in the order, propanol<methanol˜ethanol. However, exposure ofwater on the TLC plate drastically reduced the luminescence intensity.No shift in the emission wavelength was observed. The emission data arepresented in FIG. 5.

The solvent dependency of emission is currently believed to beattributed to hydrogen bonding of the solvent molecules with ligands onthe cluster. As a result, the non-radiative rate of decay will reduceand this will enhance the emission. In addition to the clustercomposition emission, FIG. 5 shows a peak at 725 nm which is attributedto the emission from the TLC plate. Asterisk (*) corresponds to regionswhere higher order line of the grating mask the spectrum and dollar ($)corresponds to the emission coming from SiO₂.

The solvent dependency of the emission was used to write letters on theTLC plate. In this approach, the TLC plate was coated with a givenconcentration of the cluster composition. In one embodiment, a lowconcentration of cluster composition was used to coat the TLC place suchthat the emission intensity is weak (and the plate is rose in color).When solvent contacts the plate, it enhances luminescence intensity andthe solvent exposed regions appear with brighter luminescence. As thesolvent evaporates, the parent luminescence reappears bringing the plateto the original state.

Example 7

At higher concentrations, QCs have a tendency to form a gel-likematerial with intense emission. Self assembly of the CD and -SGmolecules at high concentration may result in gelation. This appears tobe the first report of a gel using QCs. These materials were analyzed bySEM and HRTEM. The microstructure of the gel is composed of fibers of ˜8μm diameter (FIG. 6B). In order to study the spatial distribution ofgold in the gels formed by QCs, elemental mapping was carried out usingenergy dispersive analysis of X-rays (EDAX), FIG. 6A shows the EDAXspectrum collected from the gel shown in FIG. 6B. EDAX mapping was doneusing Au Ma and the image is given in FIG. 6C. The detailed structure ofthe fibers was examined by TEM (FIG. 6D). The isolated clusters are notseen in TEM, as mentioned before. Both in TEM and SEM, a fiber-likemorphology is observed. Cyclodextrins and their derivatives have beenextensively used as host molecules in supramolecular chemistry.Inclusion complexes (ICs) of CDs and guest molecules may result insupramolecular nanostructures (nanogels). Such complexes may findapplications in drug delivery and diagnosis, especially in view of thelow metallic content of the cluster and its high solubility in water inconjunction with luminescence.

Example 8

The cluster composition may be used for the selective detection of metalions such as Cu²⁺. An aqueous solution of the cluster composition wasprepared. Metal ions were added individually under UV light. With theaddition of Cu²⁺ ions, there was a drastic change inluminescence—disappearance of red emission followed by the emergence ofyellow emission (within a few minutes). The UV-Vis and luminescencespectra of cluster solution before and after addition of Cu²⁺ are givenin FIGS. 7A and 7B. Even after the addition of Cu²⁺, the molecularabsorption features are still intact (FIG. 7A), suggesting that thecluster composition is stable. But by looking into the PL spectra ofcluster composition before and after the addition of Cu²⁺, a drasticchange of emission maximum was observed (a blue shift of ˜100 nm).

Equivalents

While certain embodiments have been illustrated and described, it shouldbe understood that changes and modifications can be made therein inaccordance with ordinary skill in the art without departing from thetechnology in its broader aspects as defined in the following claims.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the elements (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Additionally, the terms andexpressions employed herein have been used as terms of description andnot of limitation, and there is no intention in the use of such termsand expressions of excluding any equivalents of the features shown anddescribed or portions thereof, but it is recognized that variousmodifications are possible within the scope of the invention claimed.Additionally the phrase “consisting essentially of” will be understoodto include those elements specifically recited and those additionalelements that do not materially affect the basic and novelcharacteristics of the claimed invention. The phrase “consisting of”excludes any element not specifically specified.

The present disclosure is not to be limited in terms of the particularembodiments described in this application. Many modifications andvariations can be made without departing from its spirit and scope, aswill be apparent to those skilled in the art. Functionally equivalentmethods and compositions within the scope of the disclosure, in additionto those enumerated herein, will be apparent to those skilled in the artfrom the foregoing descriptions. Such modifications and variations areintended to fall within the scope of the appended claims. The presentdisclosure is to be limited only by the terms of the appended claims,along with the full scope of equivalents to which such claims areentitled. It is to be understood that this disclosure is not limited toparticular methods, reagents, compounds compositions or biologicalsystems, which can of course vary. It is also to be understood that theterminology used herein is for the purpose of describing particularembodiments only, and is not intended to be limiting.

In addition, where features or aspects of the disclosure are describedin terms of Markush groups, those skilled in the art will recognize thatthe disclosure is also thereby described in terms of any individualmember or subgroup of members of the Markush group.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the like,include the number recited and refer to ranges which can be subsequentlybroken down into subranges as discussed above. Finally, as will beunderstood by one skilled in the art, a range includes each individualmember.

Other embodiments are set forth in the following claims.

What is claimed is:
 1. A method of preparing a quantum cluster, themethod comprising: adding a first amount of glutathione to a gold salt,a silver salt, or a mixture thereof to form a mixture; adding a reducingagent to the mixture to form a precipitate; and mixing the precipitatewith a second amount of glutathione and a cyclodextrin to form thequantum cluster; wherein the quantum cluster comprises: Ag_(m), Au_(n),or Ag_(m)Au_(n), wherein m and n are independently from 2 to 100; one ormore of the glutathione molecules bound to the Ag_(m), Au_(n), orAg_(m)Au_(n); and the cyclodextrin partially or wholly surrounding theone or more glutathione molecules bound to the Ag_(m), Au_(n), orAg_(m)Au_(n); wherein the cyclodextrin is selected from the groupconsisting of unsubstituted α-cyclodextrin, unsubstitutedβ-cyclodextrin, and unsubstituted γ-cyclodextrin.
 2. The method of claim1, having a molar ratio of the first amount of glutathione to the amountof the gold salt, the silver salt, or the mixture thereof is about 1:2to about 1:8.
 3. The method of claim 1, wherein the precipitatecomprises a) a quantum cluster of Ag_(m), Au_(n), or Ag_(m)Au_(n), andb) glutathione, wherein m and n are independently from 2 to
 100. 4. Themethod of claim 1, wherein the gold salt is a trivalent gold source. 5.The method of claim 1, wherein the reducing agent is NaBH₄ or LiBH₄. 6.The method of claim 1, wherein the step of mixing the precipitate with asecond amount of glutathione and the cyclodextrin to form the quantumcluster occurs at a temperature of about 70° C.
 7. The method of claim1, wherein the method comprises adding a first amount of glutathione toa gold salt to form a mixture; adding a reducing agent to the mixture toform a precipitate; and mixing the precipitate with a second amount ofglutathione and a cyclodextrin to form the quantum cluster; wherein thequantum cluster comprises: Au_(n), wherein n is independently from 2, 3,5, 10, or 15; one or more of the glutathione molecules bound to theAu_(n); and the cyclodextrin partially or wholly surrounding the one ormore glutathione molecules bound to the Au_(n); wherein the cyclodextrinis selected from the group consisting of unsubstituted α-cyclodextrin,unsubstituted β-cyclodextrin, and unsubstituted γ-cyclodextrin.
 8. Themethod of claim 7, wherein the step of mixing the precipitate with asecond amount of glutathione and a cyclodextrin to form the quantumcluster occurs at a temperature of about 70° C.
 9. The method of claim7, wherein in the step of adding a reducing agent to the mixture thereducing agent is NaBH₄ or LiBH₄.
 10. The method of claim 9, wherein thestep of adding the reducing agent to the mixture occurs at a temperatureof about 0° C.
 11. The method of claim 1, wherein the method comprisesadding a first amount of glutathione to a gold salt to form a mixture;adding a reducing agent to the mixture at a temperature of about 0° C.to form a precipitate; and mixing the precipitate with a second amountof glutathione and a cyclodextrin at a temperature of about 70° C. toform the quantum cluster; wherein the quantum cluster comprises: Au_(n),wherein n is 15; one or more of the glutathione molecules bound to theAu_(n); and the cyclodextrin partially or wholly surrounding the one ormore glutathione molecules bound to the Au_(n); wherein the cyclodextrinis selected from the group consisting of unsubstituted α-cyclodextrin,unsubstituted β-cyclodextrin, and unsubstituted γ-cyclodextrin.
 12. Themethod of claim 11, wherein in the step of adding a reducing agent tothe mixture the reducing agent is NaBH₄.